Acrylourethane oligomers have previously been utilized in combination with various acrylic monomers to produce radiation-curable protective coatings as well as radiation-curable vehicles for materials such as ceramic ink compositions. For example, U.S. Pat. No. 4,390,565 describes a radiation-curable ceramic ink composition containing an acrylated polycaprolactone diol polyurethane, an acrylic monomer, and a ceramic frit. U.S. Pat. No. 4,780,487 describes a radiation-curable coating composition having reduced gloss. The coating composition is a mixture of a polydiene-based acrylourethane oligomer and a standard acrylourethane oligomer. The coating composition also utilizes a reactive diluent system which can be one or more acrylic monomers.
A radiation-curable vehicle for ceramic enamels is described in U.S. Pat. No. 4,900,763. The radiation-curable vehicle contains acrylate or methacrylate modified oligomers, monofunctional acrylate or methacrylate modified monomers, pentafunctional acrylate or methacrylate modified monomers, and a photoinitiator system. The photoinitiator system is preferably a blend of a substituted thioxanthone compound, an ester of an aminobenzoic acid, and a 2-phenylacetophenone derivative.
U.S. Pat. No. 5,003,026 describes a radiation-curable no-wax floor covering which contains a polymerized urethane-acrylate oligomer which is the reaction product of an aromatic or cycloalkyl diisocyanate, a monohydroxy monoacrylate, and a phthalic polyester polyol.
U.S. Pat. Nos. 4,377,679 and 4,512,910 disclose photocurable compositions prepared by reacting a polyether diol and an acrylic acid in about equal molar proportions to form a reaction product and then reacting the reaction product with a hydroxyalkylacrylate and an organic diisocyanate in about equal molar proportions.
It has been discovered that many of the urethane-acrylate oligomers traditionally utilized in acrylourethane-based radiation-curable compositions, such as those described above, tend to be highly viscous in nature and therefore create processing problems when utilized in coating compositions. Specifically, many previous urethane-acrylate oligomers have viscosities ranging from 100,000 to 1,000,000 centipoise (cps) and therefore require substantial dilution with reactive diluent monomers in order to be utilized in radiation-curable coating compositions. Consequently, typical radiation-curable compositions contain 70 to 90 percent by weight diluent monomers.
The dilution of oligomers with reactive diluent monomers often results in an impairment of the performance of the resulting coating composition since the desirable properties of the oligomer tend to be overcome by the large amount of monomers present. For example, many of the traditional acrylourethane oligomer-based compositions do not maintain sufficient stain resistance and flexibility once the viscosity has been reduced to an acceptable level for processing. Furthermore, the reactive diluent monomers are typically very expensive and therefore substantially increase the cost of the overall radiation-curable composition. The reactive diluent monomers are also typically toxic in nature and thereby create health and environmental hazards.
A need therefore exists for a radiation-curable oligomer which can be utilized in coating compositions at viscosities low enough for acceptable processing. Such an oligomer should also maintain sufficient stain resistance and flexibility at these low viscosities.